Polyolefin microporous membranes have been widely employed, for example, as battery separators used in lithium secondary batteries, nickel-hydrogen batteries, nickel-cadmium batteries, and polymer batteries; separators used in electrolytic capacitors; a variety of filters such as reverse osmosis filtration membranes, ultrafiltration membranes, and microfilter membrane; and moisture-permeable waterproof clothes and medical materials.
In the case where a polyolefin microporous membrane is used as a battery separator, particularly as a separator of a lithium ion battery, the properties thereof have large effects on the characteristics, productivity, and safety of the battery. Hence, the polyolefin microporous membrane needs to have properties such as excellent mechanical properties, thermal resistance, appearance, permeability, dimensional stability, shutdown properties, and meltdown properties. If a polyolefin microporous membrane having a low mechanical strength is used as a battery separator, for instance, short circuit between electrodes may occur with the result that the battery voltage is lowered.
Since typical microporous membranes made of polyethylene alone generally have a low mechanical strength, a microporous membrane formed of ultra-high molecular weight polyethylene has been proposed for an enhancement in the mechanical strength.
In recent years, however, regarding the properties of separators, a demand for safety has been highly increased in addition to mechanical strength and permeability. In particular, the electrodes of a lithium ion battery repeatedly expand and contract by electric charge and discharge. In this case, shutdown properties which enable prompt shutdown of the battery circuit when a large electric current flows because of, for example, external short circuit are needed. Polyethylene microporous membranes produced by a method involving formation of pores through stretching or by a phase separation method have been practically used as the separators of lithium ion batteries; this is because the membranes are melted at a relatively low temperature owing to heat generated by the short-circuit current to close the micropores with the result that the battery circuits can be shut down, and an increase in temperature can be therefore suppressed after the micropores are closed.
In addition to closing micropores at a relatively low temperature, however, the microporous membranes used in the lithium ion batteries also need to maintain the shape thereof when the temperature is increased to a high level. In the case where the shape is not maintained, electrodes directly contact each other with the result that meltdown is caused, leading to a dangerous state. The battery separator formed of polyethylene has a low melting point and is therefore unsatisfactory in terms of the meltdown temperature. Combined use of polyolefin having a high melting point has been proposed for improvement of such meltdown properties.
In Patent Literature 1, a polyolefin microporous membrane formed of a composition containing polyethylene and polymethylpentene has been proposed. In particular, in the disclosure in Patent Literature 1, a mixture of high-density polyethylene and polymethylpentene (polyolefin resin) is melt-kneaded and then stretched at a predetermined temperature to produce the polyolefin microporous membrane; however, the produced microporous membrane has insufficient strength and thermal resistance. Furthermore, poor compatibility of high-density polyethylene with polymethylpentene causes problems in which a film is not sufficiently stretched and in which unmelted part of the resin remains to cause fisheyes with the result that the appearance of a film is impaired. Comparing part of the membrane corresponding to the residual unmelted resin (fisheyes) with the other part, pores are not sufficiently opened, and ion permeability is insufficient in the electric charge and discharge of a battery. In the case where a separator has an uneven ion permeability, not only electric charge and discharge become inefficient, but also dendrite is likely to be selectively generated at part of an electrode facing part of the separator at which ion permeability is high. If the dendrite grows and breaks the separator, external shortcut is caused, and large amount of current therefore flows, which is significantly dangerous. Hence, the fisheyes in separators need to be reduced as much as possible.
In Patent Literature 2, a microporous membrane has been proposed, in which a layered structure which includes a layer mainly containing polyethylene and a layer mainly containing polyethylene and polypropylene having a predetermined molecular weight enables development of thermal resistance. Production of the microporous membrane having a layered structure, however, costs expensive capital investment, involves a complicated production process, and is highly technical, which causes a problem in which such a microporous membrane is less likely to become popular in the market.